THE COOLING CAPACITY AND PRESSURE DROP IN A HYBRID CLOSED CIRCUIT COOLING TOWER

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1 Proceedings of the International Conference on Mechanical Engineering 25 (ICME25) 28-3 December 25, Dhaka, Bangladesh ICME5- HE COOLING CAPACIY AND PRESSURE DROP IN A HYBRID CLOSED CIRCUI COOLING OWER M. M. A. Sarker 1, C. G. Moon 2 and J. I. Yoon 2 1 Dept. of Mathematics, Bangladesh University of Engineering and echnology, Dhaka, Bangladesh masarker@math.buet.ac.bd, el: Dept. of Refrigeration and Air-Conditioning Engineering Pukyong National University, Namgu, Pusan 8-739, Korea. ABSRAC Experimental study on the Hybrid Closed Circuit Cooling ower has been done having a rated capacity of 3R. Bare-type copper coil having an outer diameter of 15.88mm has been used in the 1.14m 2.36m 3.2m dimensional tower. Heat exchanger consists of 16 rows and 22 columns. he relevant temperatures and the velocities were selected based on the typical Korean weather for the year round operation. Cooling towers reported so far operate on plume-free mode during the mid-season and winter while ambient temperature remains below 12 O C. his study presents results related to the plume free operation while the ambient temperature is less or equal to 15 O C in dry mode. he cooling capacity of the tower is explained with respect to variable temperatures, cooling water inlet temperatures and G/W ratios. he capacity was found to be close to the rated one for the wet mode operation but rather low in dry mode Keywords: Hybrid closed circuit cooling tower, Wet-bulb temperature, Cooling capacity. 1. INRODUCION he use of cooling towers to reject heat, cool buildings and reduce the temperature of water circulated through various heat rejection equipments have increased considerably in recent days [1-3]. A closed circuit cooling tower maintains an indirect contact between the fluid and the atmosphere. Cooling towers respectively are called wet tower when evaporative cooling is used, dry tower when air blast cooling is utilized and wet-dry type which has the simultaneous characteristics of both dry and wet towers. Cooling effects in wet cooling towers are partially brought about by the evaporative condenser where a quota of the circulating water gets evaporated and partially by the sensible heat transfer [4]. A number of numerical simulation [5-7] and mathematical models of the cooling tower have been reported [8-1]. Experimental studies [11-12] have been carried out on the wet cooling tower but the experimental results on hybrid closed circuit cooling tower is lacking on the relevant literature. he hybrid closed circuit cooling tower (HCCC) is a closed circuit cooling tower which is capable of working both in wet mode and in dry mode. HCCC works well in dry mode during the mid-season and winter as soon as ambient temperature is below O C, no plume and no freezing and lower noise level. HCCC operates smoothly in wet mode while the ambient temperature is above O C, water consumption by HCCC is lower and the process water can be cooled down to 4 O C above the wet bulb temperature and can be packed in light and compact bundle with optimized circuitry [13]. During the summer operation, no plume formation is expected due to the higher ambient temperature and higher dew-point temperature in Korea. In winter, only the cooled air from the bottom is used and plume-free state is predicted once more because the typical Korean winter ambient air contain lower moister. he cooling towers those are in use can operate without forming plume during the mid-season and winter only when ambient temperature is below 12 O C. he objective of this experimental study is to make HCCC which would have the ability to operate in plume free mode until 15 O C in dry mode. 2. EXPERIMENAL APPARAUS Fig. 1 shows the schematic of the hybrid closed circuit cooling tower experimental apparatus and Fig. 2 gives the photograph of the section of 3 R experimental apparatus. In the experiment, a prototype HCCC is used where the coil section is located at the upper part, fans are installed at the lower part alongside of the water tank. In the coil section, coils, spray system, eliminator are sequentially organized and is kept in a casing. he copper coil having outer diameter of 15.88mm has been used in the heat exchanger in a staggered arrangement. he cooling water is supplied by pipes having 65 mm inlet diameters and the pipe is connected to the distribution head through 44 horizontal cooling coils. ICME25 1

2 Cooling water flows in downward direction from the top to the bottom, so lower pressure drop is expected across coil bank. H P P H V Fan Cooling ower Heater Coil Flow Meter Pump B r i n e a n k Coole r Plate heat Exchanger Boiler Fig 1. Schematic of the experimental apparatus he spray system consists of the header which is made of copper tube, the spray distributing pipe and a spray nozzle having big caliber so that blocking/fouling can be prevented and thus the spray water is uniformly distributed at the upper part of the coils. 3. EXPERIMENAL MEHODS he important sequences of the experiment are as follows: After the all the relevant conditions normalized, the system were let to work for a specific time and then the data were collected and measured to minimize the experimental errors and optimize the accuracy of the outputs. A fixed air flow rate is maintained by using fan and 15HP type inverter, the discharged air from the outlet of the cooling tower which is highly humid and has high temperature, passes though the return duct and mixes with the ambient air and thereby, the dry-bulb temperature and the wet-bulb temperature are controlled. he proper air constraint is maintained by applying 1 R type heater and the cooling coil at the inlet of the HCCC. he surplus discharged air is released to the outside of the tower. Cooling water flows in the internal part of the coil of the HCCC. he cooling water after coming out through the outlet of the HCCC is sent to the plate type heat exchanger. At the other side of the plate heat exchanger, the heated water having temperature of C is supplied from the 15 Mcal/h type boiler. While passing though the plate heat exchanger, the cooling water gains heat and get stabilized to a certain temperature and is recirculated to the HCCC. o control the temperature more accurately, a bypass pipe is installed at the cooling water pipe and the heated water supplying pipe. hen the temperature of the desired level could be maintained through controlling the supply of the flow rate of the cooling water and the heated water to the plate heat exchanger. he experiment was run changing the air flow rate. he pressure loss of the air was measured when the fan functioning get stabilized and pressure difference was recorded applying manometer. he humidity sensor is used to measure the humidity and both humidity and temperature at 5 points both at the air inlet and outlet were measured at every 5 second and the averages of these values were applied. 4. RESULS AND DISCUSSION Under the standard experimental condition given in able 1, the experiment was repeated over and over again changing the wet-bulb temperature of inlet air as well as the air-water ratio. Only the valid data, selected from the stabilized state has been used for the analysis. o calculate the heat balance, following couple of equations are utilized ( ) Qr1 = W Wi Wo (1) ( G + L) ha, i G ha i Qr 2 = (2), Here, Eq. (1) gives the exchanged heat capacity of the cooling water and Eq. (2) represent the exchanged heat capacity of air brought about by the enthalpy difference between the inlet and outlet of the air. he results have been shown in Fig. 3 where the heat balance data those fall within ± 15% were used. Heat balance of the apparatus could be claimed to be satisfactory. able 1: Experimental Condition Fig 2. Photograph of experimental apparatus Cooling water Spray Water Air Volume Flow Rate [m 3 /h] 24 Inlet emperature [ O 37 Volume Flow Rate [m 3 /h] 33 Velocity [m/s] 3.1 Wet-bulb emperature [ O 27 ICME25 2

3 Q r % -15% defined in the standard design condition and the specific heat is constant, so the trend of the performance of the HCCC looks identical to that of the temperature difference range plot given earlier. At design condition, i.e. at the WB of 27 O C at the inlet of the HCCC, the cooling performance found from Fig. 5 is 112,kcal/h whereas the rated capacity is 117,kcal/h, so, the experimental performance is seen to be about 4.5% lower than the expected capacity and the difference could be claim not be that big Q r1 Fig 3. Heat balance of the experimental apparatus 4.1 Results Based on the Wet Mode Operation Effect of WB on emperature Range and the Cooling Capacity Fig.4 shows the difference of the inlet and the outlet temperatures of the cooling water with respect to the variable wet-bulb temperature (WB) of the HCCC. From this figure it is clear that the outlet temperature of the cooling water increases with the increase of the wet-bulb temperature and that at a WB of 2 O C, the outlet temperature is about 7 O C cooler than the inlet temperature. his is mainly because, when the WB at the inlet increases, then the temperature difference of the Range [ o Cooling capacity [Mcal/h] Wet-bulb temperature [ o Characteristics by Ratio of Air to Cooling Water Volume Flow Rate Fig. 6 and Fig. 7 respectively show the temperature range and the cooling capacity with respect to the air flow rate to the cooling water volume flow rate. Cooling capacity found from the ratio for the design condition is 116 Mcal/h which agrees well with the rated cooling capacity. 1 8 Fig 5. Cooling capacity w.r.t. WB Wet-bulb temperature[ o 3 32 Fig 4. emperature range w.r.t. WB Range [ o 6 4 cooling water and the air at the respective inlets decreases. herefore, the vaporization of the spray water outside of the pipe decreases so that the falling of the temperature of the cooling water flowing inside the tube deceases. he drastic decline of the temperature range could be noted while the WB is greater than or equal to 26 O C. he performance of the HCCC is shown in Fig. 5. he cooling performance of the HCCC can defined as the product of the cooling water flow rate and the temperature difference between the inlet and the outlet of the HCCC, multiplied by the specific heat of the cooling water. Since, the cooling water flow rate is G/W [-] Fig 6. emperature range w.r.t. air to cooling water flow rate. ICME25 3

4 Cooling capacity [Mcal/h] Pressure Drop with Respect to Air Velocity Pressure drop with respect to a variable air velocity has been shown in Fig. 8. Pressure drop increases almost exponentially with the increase of the air velocity. he increasing rate has been escalated due to the spray water in the wet mode operation. At the design condition, the pressure drop can be seen to be around 4 mmaq. Pressure drop [mmaq] Pressure drop [mmaq] G/W [-] Fig 7. Cooling capacity w.r.t. air to cooling water flow rate Air velocity [m/s] Fig 8. Pressure drop with respect to air velocity in wet mode Air velocity [m/s] Fig 9. Pressure drop with respect to air velocity in dry mode. 4.2 Results Based On the Dry Mode Operation Pressure Drop with Respect to Air Velocity Fig. 9 shows the pressure drop with respect to air velocity in dry mode operation. It s evident that the pressure drop increases almost linearly with the increasing air velocity. At the standard design condition, the pressure drop was around 2.3 mmaq. Cooling capacity [kcal/h] Overall heat transfer coefficient [kcal/m 2 h o Air inlet velocity of 3.1 m/s Air inlet temperature [ o Fig 1. Cooling capacity w.r.t. air inlet temperature 3 Cooling water flow rate = 24 m / h Air inlet temperature [ o 2 25 Fig 11. Overall heat transfer coefficient by air inlet temperature Characteristics with Respect to Air Inlet emperature Cooling capacity and overall heat transfer coefficient with respect to a variable air inlet temperature are respectively shown in Fig. 1 and Fig. 11. Cooling capacity decreases almost linearly with the increase of the air inlet temperature and vice versa. he cooling capacity is seen to be rather poor due to the fact that no ICME25 4

5 spray water was used during winter operation. Overall heat transfer coefficient also decreases with the increase of the air inlet temperature. 5. CONCLUDING REMARKS he performance characteristics of the hybrid closed circuit cooling tower were investigated experimentally having a rated capacity of 3R. he wet-bulb temperature at the experimental condition was 27 O C and the cooling capacity at this WB was 112, kcal/h, which is about 4.5 % lower than the rated capacity. he capacity with respect to air to the cooling water volume flow rate was 116kcal/h which agreed well with the rated capacity. All these results were obtained for the wet mode operation of the HCCC. he pressure drops in wet mode operation were seen to increases almost exponentially with the increase of the air velocity and the pressure drop was about 4 mmaq for the wet mode. Performance characteristics have also been investigated for the dry mode operation. At an air inlet temperature of 1 O C, the cooling capacity was about 43 kcal/h which is around 36% of the rated capacity. At the design condition, the pressure drop for the dry mode was around 2.3 mmaq. hese lower capacities are due to the absence of the spray water. It may be mentioned that this apparently lower performance is cost-effective due to the lower power consumption. he result obtained from this study is supposed to provide basic relevant data which could be referred for the optimum design of the hybrid closed circuit cooling towers. 6. REFERENCES 1. ES Engineering system, esmagazine.com 2. Cooling towers, he Marley cooling tower company, Handbook of HVAC Design 199, McGraw-Hill, Inc. 3. Ala Hasan and Kai Siren, 22, heoretical and computational analysis of closed wet cooling towers and its applications in cooling of buildings, Energy and Buildings, 34(5) Cooling towers, 2, ASHRAE HVAC Systems and Equipment, American Society of Heating, Refrigeration and Air Conditioning Engineers, USA. 5. G. Gan, S.B. Riffat, 1999, Numerical simulation of closed wet cooling towers for chilled ceiling systems, Applied hermal Engineering 19 (12) G. Gan, S.B. Riffat, L. Shao, P. Doherty, 21, Application of CFD to closed-wet cooling towers, Applied hermal Engineering, 21 (1) Gan, G. ; Riffat, S.B., 1999, Predicting thermal performance of a closed-wet cooling tower for chilled ceilings, Building services engineering research & technology 2 (4) Jameel-Ur-Rehman Khan, M. Yaqub, Syed M. Zubair, 23, Performance characteristics of counter flow wet cooling towers, Energy Conversion and Management M.S. Soylemez, 21, On the optimum sizing of cooling towers, Energy Conversion and Management, 42 (7), Korenic, B., 1995, Computerized Simulation of Closed Circuit Cooling owers with Parallel and Counterflow Spray Water-Air Flow Design, CI journal, P S. V. Bedekar, P. Nithiarasu, K. N. Seetharamuz, 1998, Experimental Investigation Of he Performance Of A Counter-Flow, Packed-Bed Mechanical Cooling ower, Energy 23 (11) Nenand Milosavljevic, Petti Heikkilia, 21, A comprehensive approach to cooling tower design, Applied thermal Engineering, 21 (9), GEA erge-spirale soramat, 7. NOMENCLAURE Symbol Meaning Unit G Flow rate of air (kg/h) h Enthalpy (kcal/kg.dry air) L Portion of spray water lost due to evaporation (kg/h) Q Cooling capacity (Kcal/h) W Cooling water flow rate (kg/hr) ACKNOWLEDGEMEN his study was supported financially by the Korea Science and Engineering Foundation through the Center for Advanced Environment Friendly Energy Systems, Pukyong National University, Korea. heir cooperation is greatly acknowledged. ICME25 5

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